Environmental stress has played an enormous role in the evolution and physiology of organisms over time. Microbial ecosystems in particular have responded to environmental stress by developing complex biofilm community structures and novel gene products. The aim of this proposal is to explore the role of oxidative stress in nature by examining global regulatory mechanisms in microbial mat communities using newly developed genetic techniques. More specifically, I propose to: 1) determine exogenous and endogenous levels of reactive oxygen species in cultured cyanobacterium and in field collected microbial mat samples; 2) determine the effects of reactive oxygen species on the growth of the cultured cyanobacterium Synechocystis sp 6803, and in situ with field collected microbial mat samples; and 3) identify biofilm associated genes regulated by oxidative stress using a DNA microarray designed to the cyanobacterium Synechocystis sp 6803. Taken together these studies would represent the first study to examine the global gene regulatory response of microbial mat biofilms exposed to oxidative stress in situ.